Fibrosis is estimated to contribute, either directly or indirectly, to nearly 45% of deaths in the developed world. Fibrotic diseases are characterized by an excessive accumulation of extracellular matrix components, which disrupts the physiological tissue architecture, leading to the dysfunction of the affected organ. In some instances, fibrosis is thought to be a consequence of chronic tissue irritation or chronic inflammation. In some instances, fibrosis is thought to be a consequence of autoimmune reactions within the body. The progressive replacement of parenchymal tissues with extracellular matrix components is observed in fibrotic diseases such as scleroderma, pulmonary fibrosis, and liver cirrhosis. However, the cellular and molecular factors that sustain the fibrotic cascade remain poorly understood.
Wnt-signaling has been implicated in several human fibrotic diseases. Activated Wnt-signaling in some types of fibrotic diseases may be identified by accumulation of β-catenin, up-regulation of Wnt pathway ligands such as Wnt1 and Wnt10B, and down-regulation of Wnt pathway negative regulators such as Dkk1 (Akhmetshina et al., 2012, Nature Communications, 3:735; Guo et al., 2012, Physiol. Res., 61:337-346). The Wnt signaling pathway is one of several critical regulators of embryonic pattern formation, post-embryonic tissue maintenance, and stem cell biology. Unregulated activation of the Wnt pathway is associated with many human diseases where pathogenesis may proceed by overtaking homeostatic mechanisms which control normal development and/or tissue repair.
Wnt ligands and R-spondin (RSPO) proteins have been shown to synergize to activate the canonical Wnt pathway. RSPO proteins are known to activate β-catenin signaling similar to Wnt signaling, however the relationship between RSPO proteins and Wnt signaling is still being investigated. It has been reported that RSPO proteins possess a positive modulatory activity on Wnt ligands (Nam et al., 2006, JBC 281:13247-57). This study also reported that RSPO proteins could function as Frizzled8 and LRP6 receptor ligands and induce β-catenin signaling (Nam et al., 2006, JBC 281:13247-57). Recent studies have identified an interaction between RSPO proteins and LGR (leucine-rich repeat containing, G protein-coupler receptor) proteins, such as LGR5 (U.S. Patent Publication Nos. 2009/0074782 and 2009/0191205), and these data present an alternative pathway for the activation of β-catenin signaling.
The R-Spondin (RSPO) family of proteins is conserved among vertebrates and comprises four members, RSPO1, RSPO2, RSPO3, and RSPO4. These proteins have been referred to by a variety of names, including roof plate-specific spondins, hPWTSR (hRSPO3), THS2D (RSPO3), Cristin 1-4, and Futrin 1-4. The RSPOs are small secreted proteins that overall share approximately 40-60% sequence homology and domain organization. All RSPO proteins contain two furin-like cysteine-rich domains at the N-terminus followed by a thrombospondin domain and a basic charged C-terminal tail (Kim et al., 2006, Cell Cycle, 5:23-26).
Studies have shown that RSPO proteins have a role during vertebrate development (Kamata et al., 2004, Biochim. Biophys Acta, 1676:51-62) and in Xenopus myogenesis (Kazanskaya et al., 2004, Dev. Cell, 7:525-534). RSPO1 has also been shown to function as a potent mitogen for gastrointestinal epithelial cells (Kim et al., 2005, Science, 309:1256-1259). It has been reported that RSPO3 is prominently expressed in or close by endothelial cells and their cellular precursors in Xenopus and mouse. Furthermore, it has been suggested that RSPO3 may act as an angiogenic factor in embryogenesis (Kazanskaya et al., 2008, Development, 135:3655-3664).
Drug treatment options for patients diagnosed with a fibrotic disease are very limited. There is a need for new agents targeting fibrosis, and signaling pathways involved in fibrosis. Thus, biomolecules such as RSPO-binding agents that disrupt signaling pathways involved in fibrosis are a potential source of new therapeutic agents.